Tuning of mass flow controllers is typically performed by the MFC manufacturer with a calibration gas, typically Nitrogen (N2). Such tuning may be performed in order to develop one or more algorithm parameter coefficients. These parameter coefficients are adapted to be applied to raw MFC data so that the data more accurately reflects the actual flow conditions in the MFC, as compared to the raw MFC data. Often, the obtained parameter coefficients are gas-dependent. Therefore, when the MFC is operating with a gas other than N2, the coefficients may output inaccurate and/or delayed results.
In one such case where a MFC is operating with a non-tuning gas, the MFC may receive a command for a zero setpoint after providing a non-zero setpoint of flow. In such a case, the valve is closed substantially immediately upon receiving such a command, resulting in no net gas flow through the MFC. However, although there is no net gas flow through the MFC itself, the thermal flow sensor is slow in responding to changes in gas flow rates, due to the nature of heat redistribution inside the sensor, the non-zero mass of sensor components, etc. Therefore, the MFC thermal flow sensor provides an inaccurate output that flow is still occurring in the MFC after the zero setpoint command is received. In order to correct for the inaccurate output, one or more digital filters comprising the algorithm parameter coefficients are implemented. As these parameters are calculated during the MFC tuning step described above, the output is only properly corrected for N2 gas. For gases other than N2, the corrected output may overshoot and/or undershoot the zero setpoint. Furthermore, for non-manufacturing-tuning-gasses, the thermal flow sensor may continue to provide output for a substantial period of time after receiving the zero setpoint value, resulting in the output waveform having a “long tail”. Due to the inaccurate sensor response, similar errors will be present at other setpoint-to-setpoint flow transitions.
Similarly, when MFC inlet pressure changes, the pressure change may produce a “parasitic flow” in the MFC. Parasitic flow comprises a flow that is internal to the MFC—flowing from a MFC inlet portion to a MFC “dead volume” located between the bypass and the MFC valve. To correct for any flow rate output due to the parasitic flow, data from a MFC pressure sensor and the thermal flow sensor may be used to obtain a MFC correction algorithm comprising one or more digital filters. The correction algorithm may estimate the parasitic flow that is caused by the gas pressure deviation and calculate actual flow rate in the MFC. However, the default parameters of the parasitic flow correction algorithm are obtained using N2 during manufacturer's tuning. Therefore, in MFC processes that use gases other than N2, the parameters in the parasitic flow correction algorithm will not produce accurate flow rate readings. Therefore, in such cases, there is an increase in MFC pressure sensitivity.